uarea related cleanup

Hello,
To finish uarea clean-up and remove struct user (and sys/user.h), I would
like to make few changes in our MD code:
- Rename lwp_t::l_addr to l_pcb, and replace USER_TO_UAREA/UAREA_TO_USER
et al macros with uvm_lwp_getuarea() and uvm_lwp_setuarea() routines.
- Minimise direct access of lwp_t::l_addr/l_pcb by replacing them with a new
lwp_getpcb(lwp_t *l) routine. Mainly assembler would use l_pcb directly.
This should give a slightly better abstraction and small simplification.
Comments?
--
Mindaugas